1. Field of the Invention
This invention relates generally to disk drive parking control systems and, more specifically, to the means by which such systems park the disk drive heads using power derived from the kinetic energy of the spinning disk when external power is removed from the disk drive.
2. Description of the Related Art
A computer typically includes one or more hard disk drives that provide economical, nonvolatile storage for relatively large quantities of data. A typical hard disk drive includes one or more platters or disks having magnetic recording surfaces, disk drive controller electronics, one or more actuator arms on which are mounted magnetic transducer heads, an actuator motor for moving the actuator arms, and a spin motor for rotating the disks. The disk drive may also include amplifier and driver circuitry for the heads and motors, respectively. A well-known type of actuator motor is known as a voice coil motor (VCM). In response to signals received from the controller, the VCM swings an actuator arm and its transducer heads across the surface of the disk. The procedure of moving the heads to a predetermined position on the disk (at which it is usually desired to read or write data) is known as a seek.
A seek may use a one-pass control procedure that merely sends the heads to a predetermined disk location without processing feedback signals or it may use an iterative control procedure that iteratively adjusts the seek motion responsive to feedback of true head position information. In the iterative process, the controller receives servo signals from the heads and uses those signals to determine the radial position of the heads on the disk. In response to this servo feedback, the controller adjusts the VCM drive signal to first quickly move the heads to the desired radial position and then to maintain the heads at that position. The controller accesses data organized in concentric annular tracks on the disks by positioning the heads at radial positions corresponding to the tracks. Data are stored in sectors on each track. With the heads hovering over the target track on the spinning disks, the controller may activate the heads to record data on the disks or read data from the disks.
The controller electronics typically include a microprocessor or microcontroller that operates in accordance with firmware instructions. The microprocessor controls the VCM in response to read and write commands it receives from the host computer. The microprocessor controls the VCM by providing digital acceleration values to a digital-to-analog converter (DAC), the output current of which is provided to the VCM driver. This DAC is known as the demand DAC.
The VCM operates in response to current through its coil. To efficiently seek the heads from an initial track to a target track, the microprocessor increases the current provided to the VCM to accelerate the actuator as it leaves the initial track and then reverses the current provided to the VCM to decelerate the actuator as it approaches the target track.
A disk drive typically includes a means for moving the actuator arm to a band or area at one edge of the disk outside of the area reserved for data storage. This procedure is known as parking. The area in which the heads are parked is most typically at the extreme inside edge of the disk and is known as the parking zone. Parking the heads minimizes the likelihood that the heads will inadvertently contact and thus damage the magnetic recording surface of the data storage portion of the disk.
A disk drive may park the heads in response to a park command received from the host computer. One of two different methods may be used to park the heads. A controlled seek to the landing zone under microprocessor control may be employed or a "hardware parking sequence" can be invoked. A useful hardware parking sequence puts the actuator driver into a controlled output-voltage mode to produce a low output voltage having a polarity and value sufficient to move the actuator toward the landing zone at low speed. This moves the heads slowly across the disk until they are stopped in the parking zone when the actuator arm hits a "crash stop" member. The crash stop absorbs the (slight) impact force to decrease the likelihood of damaging the heads or other portions of the actuator assembly.
A disk drive may also automatically park the heads in response to a power failure. A disk drive typically receives its power from the host computer direct current (DC) power supply via a cable. If a user turns the host computer off, or if the disk drive loses host computer power for any other reason, it is desirable to automatically, i.e., without user intervention, park the heads. A disk drive that parks its heads in response to a power failure typically includes a circuit for detecting failing host computer DC power, a circuit for rectifying the back electromotive force (BEMF) produced by the spinning disk, and a hardware parking circuit for implementing the hardware parking sequence mentioned above. Because host computer DC power is unavailable or of too low voltage to power the demand DAC, the rectifier circuit converts the BEMF to DC and supplies it to the actuator driver during parking. The kinetic energy stored in the still-spinning disk after power has been lost is sufficient to park the heads in this manner. A disk drive also typically includes a circuit for braking the disk by shorting the spindle motor windings after a delay period sufficient to ensure that the heads have reached the parking zone.
Parking the heads in response to detection of a power failure by using a constant voltage from the actuator driver is inefficient because maximum parking velocities and/or maximum seek velocities must be limited to handle a worst-case parking scenario. In the worst-case scenario, if a seek in the direction toward the parking zone is in progress when power fails, the actuator may not decelerate significantly when the reduced predetermined voltage is applied to the VCM because of the length of the VCM electrical time constant. The actuator arm may therefore hit the crash stop at a velocity higher than intended in the hardware parking sequence design. The maximum parking velocity and/or maximum seek velocity must be limited to a value below the maximum velocity of which the actuator is capable to eliminate a requirement for undesirably larger or more resilient crash stops to prevent damage to the heads and actuator from crash stop collisions at higher velocities. Larger or more complex crash stops are uneconomical and decrease the disk surface area usable for data storage.
It is desirable to provide a system for parking the heads in response to detection of a power failure that does not require limiting the maximum seek velocity and does not decrease the area of the disk surface usable for data storage. These problems and deficiencies are clearly felt in the art and are solved by this invention in the manner described below.